DNA is made of four deoxyribonucleoside triphosphates, provided by the de novo and the salvage pathway. The key enzyme of the de novo pathway is ribonucleotide reductase, which catalyses the reduction of the 2′-OH group of the nucleoside diphosphates, and the key salvage enzymes are the deoxyribonucleoside kinases, which phosphorylate deoxyribonucleosides to the corresponding deoxyribonucleoside monophosphates.
Deoxyribonucleoside kinases from various organisms differ in their substrate specificity, regulation of gene expression and cellular localisation. In mammalian cells there are four enzymes with overlapping specificities, the thymidine kinases 1 (TK1) and 2 (TK2), deoxycytidine kinase (dCK) and deoxyguanosine kinase (dGK), which phosphorylate purine and pyrimidine deoxyribonucleosides. TK1 and TK2 are pyrimidine specific and phosphorylate deoxyuridine (dUrd) and thymidine (dThd), and TK2 also phosphorylates deoxycytidine (dCyd). dCK phosphorylates dCyd, deoxyadenosine (dAdo) and deoxyguanosine (dGuo), but not dThd. dGK phosphorylates dGuo and dAdo. TK1 and dCK are cytosolic, and TK2 and dGK are localised in the mitochondria, although recent reports indicate a cytoplasmic localisation of TK2 as well.
Based on homology to a thymidine kinase derived from a Myxoma virus, a gene from rice encoding a thymidine kinase has been proposed [Hemayet Ullah, Dominique Robertson, and Roger C. Fites: A Gene for Thymidine Kinase in Plants (Accession No. AF066050; Plant gene register PGR99-048) Plant Physiol. 1999 119 1567]. However, only a partial sequence was isolated, which sequence is not sufficient for expression of the active protein.
Two pieces of genomic DNA from Arabidopsis thaliana have been annotated as putative thymidine kinases in GenBank™ (Accession Nos. AAF13097 and BAB09824). However, to this date no experimental work towards characterisation, properties, localisation, use or biological function of plant kinases has yet been accomplished.
AZT (3′-azido-3′-deoxythymidine, Zidovudine, Retrovir®) is a nucleoside analog used in the treatment of HIV-infections. The rate-limiting step of its activation in human cells is the activation of AZT-monophosphate (AZTMP) to AZT-diphosphate.
It has been suggested to use human Herpes simplex virus type 1 thymidine kinase (HSV1-TK), having an endogenous thymidine monophosphate kinase activity, for gene therapy treatment of HIV-infections. HSV1-TK is phylogenetically related to human TK2 but not to human TK1. Also the use of HSV1-TK in order to improve the antiviral activity of zidovudine has been suggested, and the use of HSV1-TK in the combination with AZT has shown to be able to kill transformed E. coli bacteria. However, because it is believed that the phoshorylation of AZTMP is the rate limiting step in AZT activation in humans, no experimental work towards an effective combination of AZT and a thymidine kinase for use in the treatment of human cancer or in other human abnormal cell growth related diseases has been accomplished.